The conventional X-ray CT (Computed Tomography) apparatus images subjects on the basis of preliminarily set scan conditions. The imaging is executed by emitting X-ray towards the subject and reconstructing signals based on detecting X-ray passed through the subject. Normally, when performing X-ray CT examinations, one examination consists of a plurality of elements, each of which is set for a different body part and has a respective scan condition which reconstruction may be different on an element to element basis. Thus, by reconstruction element is meant units of scan action executed by a single scan condition.
When setting scan conditions, a scan plan assistance apparatus, designed to assist operation of scan planning, is generally used. One scan plan assistance apparatus preliminarily displays a expected value of exposure dose in accordance with a set scan condition (For example, Japanese patent disclosure (Kokai) No. 2005-185549). As the barometer indicating exposure dose, for example, CTDI (Computed Tomography Dose Index) and DLP (Dose Length Product) are used.
The operator can decide whether the planned exposure dose is excessive or not by comparing the exposure dose barometer with reference to an exposure dose value recommended by ICRP (International Commission on Radiological Protection) or IEC (International Electrotechnical Commission).
However, in the actual CT imaging, confirmation of the exposure dose is not necessary and a problem may arise that excess X-rays may be radiated towards the subject. In X-ray CT diagnosis, restraint of exposure dose is a big problem but actual exposure dose decisions are entrusted to operators as mentioned above. Therefore, nowadays, raising operator awareness of exposure dose of an X-ray CT apparatus operator has been a problem.
Furthermore, introduction of area detectors aggravates the problem because opportunities of repeat scan toward same part have increased. Especially, in the case of scanning a patient at risk to radiation, for example, a child, sufficient care is necessary.